The present invention relates generally to the field of printing apparatus, and more particularly to devices and methods for moving and metering marking material in such devices.
There are a variety of marking systems currently known which utilize ejection of liquid inks for marking a substrate. Ink jet and acoustic ink ejection are two common examples. Systems ejecting liquid inks present several problems as the spot size is decreased, such as when designing to increase the resolution of a printer. For example, to produce a smaller spot on a substrate, the cross-sectional area of the channel and/or orifice through which the ink must be ejected is decreased. Below a certain cross-sectional area, viscosity inhibits proper flow of the ink, adversely affecting spot position control, spot size control, etc. Thus, there has been proposed apparatus for marking by ejecting a dry or solid, particulate marking material (hereafter particulate marking material), for example the ballistic aerosol marking apparatus of the aforementioned U.S. patent application Ser. No. 09/163,893.
One problem encountered with the use of particulate marking material is in the transport of that material from a reservoir holding such material to the point of delivery. With liquid inks, the material may flow through a channel or the like. However, particulate material tends not to flow, tends to clog, and otherwise may require transport augmentation.
Another problem encountered with the use of particulate marking material is in the metering of the material for delivery to a substrate. In order to enable proper spot size control, grey scale marking, and the like, it is necessary to introduce a precisely controlled, or metered amount of marking material, at a precisely controlled rate, and at a precisely controlled time for delivery to the substrate.
In U.S. Pat. 5,717,986, it is suggested that a grid of interdigitated electrodes may be employed, in conjunction with external driving circuitry, to generate an electrostatic traveling wave, which wave may transport toner particles from a sump to a latent image retention surface (e.g., a photoreceptor) for development. The system is relatively large, and as described, applies to a flexible donor belt used in ionographic or electrophotographic imaging and printing apparatus. As described, it is not suited to application in a particle ejection-type printing apparatus, as will be further described.
Traveling waves have been employed for transporting toner particles in a development system, for example as taught in U.S. patent Ser. No. 4,647,179, which is hereby incorporated by reference. According to said patent, the traveling wave is generated by alternating voltages of three or more phases applied to a linear array of conductors placed about the periphery of a conveyor. The force F for moving the toner about the conveyor is given by F=Q.multidot.E.sub.t, where Q is the charge on the toner particles, and E.sub.t is the tangential field supplied by a multi-phase a.c. voltage applied to the array of conductors. Toner is presented to the conveyor by means of a magnetic brush, which is rotated in the same direction as the traveling wave. This gives an initial velocity to the toner particles which enables toner having a relatively lower charge to be propelled by the wave. Again, as described, this approach is not suited to application in a particle ejection-type printing apparatus, as will be further described.